1. Field of the Invention
The invention relates to a hydraulic control apparatus for a hydraulic power transmission with a lock-up clutch for a vehicle.
2. Description of Related Art
There are vehicles provided with a hydraulic power transmission with a lock-up clutch, such as a torque converter with a lock-up clutch or a fluid coupling with a lock-up clutch. Hydraulic control apparatuses for such hydraulic power transmissions with a lock-up clutch have been proposed. For example, the hydraulic control apparatuses may direct the lock-up clutch to slip to absorb periodic torque fluctuations of an engine when a vehicle runs at low speeds. Japanese Patent Application Publication No. JP-A-5-99331 describes an example of such hydraulic control apparatuses. The hydraulic control apparatus described includes a lock-up relay valve; a lock-up control valve; and a linear solenoid valve. The lock-up relay valve is switched between two positions in response to the operation of a switching electromagnetic valve such that the lock-up clutch is engaged or disengaged. The lock-up control valve adjusts the pressure of hydraulic oil flowing from the hydraulic power transmission when the lock-up relay valve is switched to the engaged position, thereby controlling the engagement of the lock-up clutch. The linear solenoid valve outputs a signal pressure applied to a spool valve element to control the lock-up control valve. That is, the lock-up control valve controls the engagement, the disengagement, and the slip state of the lock-up clutch.
As described above, the conventional hydraulic control apparatus for a hydraulic power transmission with a lock-up clutch for a vehicle includes the lock-up relay valve and the lock-up control valve in addition to the switching electromagnetic valve and the linear solenoid valve. Each of the lock-up relay valve and the lock-up control valve includes a spool valve element. Therefore, the configuration of the hydraulic control apparatus is complex, which increases size of the hydraulic control apparatus.